The invention relates to air or water current meters and, in particular, to vector measuring meters of a mechanical type employing fan-like flow sensors.
The present meter primarily has been developed for use in the upper ocean to operate in the presence of high frequency horizontal and vertical velocity fluctuations associated with surface waves and mooring motions without introducing errors into the measurements. However, operation in other water environments or in air also are contemplated.
Most current meters use electromagnetic, acoustic or mechanical sensors. Electromagentic types generate a local magnetic field and sense the electric field induced by the passage of a conductive fluid, such as sea water, through the magnetic field. Their sensors may be designed to measure orthogonal components of flow or they may have one sensor to measure speed and a vane or fin to align the instrument with the direction of flow. Acoustic sensors are of different types one of which measures Doppler shift of back-scattered sound while another measures the difference in travel time of acoustic signals sent in opposite directions along the same path. Usually the acoustic sensors are mounted in pairs at right angles to measure orthogonal components. Mechanical current meters use an S-shaped rotor or propeller as a speed sensor plus a vane as a direction sensor. Some of these instruments are comparable to the present meter in that they mount their propellers at right angles to measure orthogonal components.
The various types of meters also employ various sensing techniques which, generally, can be divided into two distinct groups one of which senses speed and direction with horizontal velocity being measured in polar coordinates. The other senses components of velocity in Cartesian coordinates, using flow sensors mounted at right angles. In both groups an internal compass establishes the orientation of the instrument with respect to magnetic north. However, regardless of the nature of the sensing technique, most meters are designed to remain in the ocean for relatively long periods of time. Consequently, for practical reasons, they use a periodic sampling procedure other than a continuous one. Such sampling itself can introduce measurement errors that are seriously compounded if the sensor response is not accurate or, in other words, not a true cosine response.
In undertaking the present development, all known types of current meters were investigated and, for a variety of reasons, the goal became one of developing a mechanical type of meter using fan-like sensors or propellers having a true cosine response. Electromagnetic sensors were not considered appropriate since they have a tendency to over respond in unsteady flow conditions. Additionally, their performance depends rather critically on the boundary layer flow around the sensor. Acoustic sensors, in turn, require sophisticated electronics and their design is considerably more complex and expensive.
As to the prior art mechanical propellers, there are some which have been designed for and used in the upper ocean. In general, however, no prior art propeller sensor or sensor arrangement has been found which is capable of providing the requisite cosine response accuracy particularly under the contemplated upper ocean environment for which the present arrangement primarily is intended.
The principal object of the invention, as should be apparent from the foregoing description is, generally, to provide an accurate cosine response of a propeller-type flow sensor. A further object is to provide the true cosine response in a meter having a pair of flow sensors mounted at right angles to measure orthogonal components. These objects are achieved primarily by employing two sensors set at right angles with each sensor formed of back-to-back true cosine propellers.